xrandr settings in CLI are reset every 60 seconds

I'm stuck with this command : xrandr --output HDMI-1 --brightness 0.50.

I began a few days ago to connect my laptop to an external monitor. This monitor is an old LCD, so I have to let the brightness setting of the monitor over 95% so that it does not become too noisy. And the monitor is then clearly too bright for my eyes :(

I've search for a workaround and found this xrandr command very usefull to reduce the brightness : it worked well one or two days, and then, without any notice, the monitor (or the system, I don't know ... presumably rather the system, of course) lost this setting! I re-entered it, but it lost it 60 seconds after, so I re-entered it, and ... again and again! Rebooting fixed it once (I mean, for a few hours), but it came back anew.

So I will reboot to fix it anew, but it's obviously sadening and not normal. I've searched for config files for xrandr and did not found them (I don't know neither where this command inputs : any Xorg file?). I tried ARandR program but it does not seem to drive this peculiar setting. I installed xbacklight but the command xbacklight =50 doesn't seem to work -though it calls xrandr also?! I don't see anything in X.org log files, nor in syslog files (with 60' interval).

Ho, it seems it's not 60', it's because I've retried the command a few seconds after reset : it is fixed to a certain timing (currently everytime the time is xx:xx:51, hence my belief it was every 60' : it will be ten seconds before rest if I launch the command at xx:xx:41 !!).

So, I'm rebooting (and scripting the xrandr command in my bashrc to ensure it at login), but I would really appreciate any help or hint on where to search, as it is very painfull.

Thanks for any help :)

[EDIT] I've tried logout/login, it doesn't work either.

Now reboot does not work any more since yesterday :(

I've parsed deeply all the logs in /var/log, found nothing.

If I recall the xrandr command very quickly after reset, reset comes anew, for a few seconds I can drill xrandr command/reset (until 4 times is the maximum), so it seems the system needs a few second to "record" its state?

If I change the lid's position (going opened from closed, or vice-versa), the monitor is reset, as well as unplugging the HDMI adapter or the VGA adapter (as my external monitor is VGA only).

In the end yesterday before a second reboot (as the first did nothing), by closing the opened applications the xrandr command remained active (bypassing the XX timing that should have reset it), so it maybe deals with some applications, or ... a "daily" timer? (I haven't recorded the exact time, it seems to happen at evening).

I'm still investigating, but any hint would be appreciated :)

[EDIT] "it seems to happen at evening" ... I think I got it : I can reproduce the problem with the "night light" parameter! I'm looking into manual timer and current timer (was set to 'sunrise'), this should be fixed soon.